Low-angle radiation antenna



Patented Mar. 28, 1950 UNITED STATES PATENT OFFICE LOW-ANGLE n-AmAT'io ANTENNA Charles L. jefiers, San Antonio, Tex.

Application March 2, 1948, Serial No. 12,592

G'Clainls. 1

This invention. relates to a radio antenna system especially useful as a broadcasting antenna, although it is: not limited to thisuse.

An object of the invention is to devise an antenna system which radiates very little energy over a rather wide high-angle range andat the same time provides a substantial gain in signal strength over the usual sing-1e vertical antenna.

Still another object of th invention is to devise an antenna system in which the angle of minimum radiated energy can be varied electrically from. 40 to 60 with. respect to' the horizontal, thus permitting adjustment of the fading wall for maximum primary service.

The above objects and others are attained by forming the antenna system oftwo vertically aligned. sections which are excited by in-phase currents ofpredetermined relative magnitudes, and the dimensions of the two antenna sections are chosen. to reduce the high-angle radiation and increase the low-angle radiation.-

My invention will be described in. connection with the accompanying drawingin which Figure I is a diagrammatic representation ofthe antenna system;

Figures 2 and 3 are radiation curvesfor the different components of the antenna system;

Figure 4 is a diagrammatic showing of one specific form of antenna system according to my invention:

Figure 5 is a diagrammatic showing of a second form of the antenna system; and

Figure 8 shows three radiation curves for di'f fe'rent current ratios in the two sections of the antenna. The primary service area of a broadcast station is important from a service and a commercial point of view. The ideal antenna for a high powered broadcast station, providing both a groundwave and a" skywave service, should have The power, frequency, and the ground conductivity around the individual station, determine the signal strength at a given location, while the general noise level fixes the minimum signal in tensity required to provide a satisfactory service. Thus, the limit of satisfactory service is" unique 2 with each individual station; and a definite vertical radiation characteristic is required to achieve the ideal in each case. p

In recent years, considerable work has been done towards improving the vertical radiation characteristics of broadcast antennas, and today a tower of uniform cross-section and approximately 0.53 wavelength high is generally used to secure the maximum nonfading range. This antenna is still far from ideal since the distortion zone usually limits the primar service area at night.- This undesirable limitation is caused by the radiation of considerable power at the higher vertical angles. 7

My improved antenna system radiates practically all energy at angles below 50 degrees of the horizontal and extremely little energy above this elevation. Further, this angle can be varied from 40 to degrees by a simple electrical adjustment with but a small increase in higher angle radiation These limits are equivalent to distances of to 160 miles for the first reflection skywave signal, fixing the location of the center of the fading zone at approximately to 200 miles from the transmitter.

My antenna system broadly consists of two vertical elements, one directly above the other as shown in Fig. 1. It is apparent that this is a combinationof an antenna I at ground elevation and an antennaZ elevated above the earth. The vertical radiation characteristics of each of these antenna types are well known. It is the unique combination of these two antennas into a single radiator that reduces the high angle radiation.

In Fig. I, the section" I at ground elevation has a length- A; and the elevated section 2 has a length B- with the center of this. section at a height H above ground. The dimensions A, B and may be expressed in degrees of the wavelengthx in free space'as follows:

A=360 (ct/X) degrees 5:360 (b/x) degrees H=360 ii/M degrees where a, 5 and h are the corresponding distances expressed in the same unit of length as A.

The antenna sections are excited or driven so that the loop currents in elements I and 2 are in phase. If the magnitude of the loop current in I is considered as unity, the currentra'tio can be expressed-as the vertical radiation characteristic for the lower section I is KAJ() =[cos (A sin 6) cos Al/cos 0 (2) where 0 is the angle above the horizontal. For the upper section 2, which for simplicity in calculations is considered a half-Wave element, the vertical characteristic is KBf 6 =2m cos (so sin 0) cos (H sin 0) /cos a (3) and for both sections, adding (2) and (3), we have Kof(9) =[cos (a sin 0) cos Al/cos 0+ 2m cos (90 sin 0) cos (H sin 0) /cos 0 (4) The form factor, Kc, referred to the current loop is 1-cos A+2m and the vertical radiation characteristic for the antenna is cos (A sin 6) cos A+2m cos (90 sin 6) cos (Hsin 0) (l-cos A+2m) cos 0 eifect of the denominator of Equation 5 in developing the optimum design of an antenna to give a particular radiation characteristic. This is done by evaluating separately the first part of the numerator of (5) for K0 cos 0f(0)A=cos (A sin 0)-cos A and the second part of the numerator for K0 cos 0,f(0)B/m=2 cos (90 sin 0) cos (H sin 0) (7) Fig. 2 is a plot of Equation 6, K0 cos 0 times the A vertical radiation characteristics of the lower section I for various lengths A, while Fig. 3 is a similar plot of Equation '7 for the upper section 2 for certain center heights II.

In Fig. 2 curves 6, l and 8 show the characteristics for three different lengths of lower section I, that is, for A values of 140, 120 and 100, respectively. On Fig. 3 curves 3, 4 and 5 show the characteristics for three different heights of the section 2, that is, for H values of 190, 210 and 230, respectively.

An examination of Fig. 3 shows, for the values of H considered, that the field from the elevated element goes through a phase reversal around degrees, and above this point will be 180 degrees out of phase with the high angle field from the lower element. If the parameters A and H are correctly selected and if the loop current in the elevated element is made less than the current in the lower element by the proper amount, the fields of the two elements can be made nearly equal and will cancel for angles deviating considerably from the vertical.

Figure 6 shows the radiation characteristic of the combined radiation from both vertical sec- A tions of the antenna system, for three diiferent current ratios. For curve 40, the current ratio m is 0.9, for curve the ratio is 0.69, and for curve the ratio is 0.6. These curves are for dimensions of A=120, B=1 and H=210.

From Figure 6 it will be seen that the angle of zero radiation may be shifted from 40 to 60 degrees simply by changing the current ratio in the two antenna sections, and this involves only an electrical adjustment.

It is of interest to note from Figure 6 that for the design of zero radiation at 50 degrees, the maximum high-angle radiation occurs at 56 degrees where it is less than 0.02 of that in the horizontal plane.

The theoretical field gain of my antenna, for an m ratio of 0.69, is 14.5% compared to a 0.53 wavelength antenna, or 41.5% referred to a quarter wavelength antenna. These field gains are equivalent to power gains of 32 and per cent respectively.

The advantage of my antenna system may be shown by comparing its performance with that of a 0.53 wavelength antenna. Both antennas are assumed to radiate a field of 1770 millivolts per meter at one mile on a frequency of 1000 kilocycles over earth with a conductivity of 10- E. M. U. The skywave signal values are for 50% of the time. For the 0.53 antenna, the center of the fading zone is at 127 miles where the groundwave signal has an intensity of 0.6 mv./m. My antenna has the same distortion point at 164 miles and at a signal intensity of 0.29 mv./m. The width of the tWo-to-one signal ratio skywave and groundwave fading zone has been reduced from 36 miles to 24. miles because of the difference in the angle of the skywave signal rise. In other words, the distortion zone of bad fading has been moved from a point of good rural primary service to a point where the groundwave signal strength is hardly adequate for year round noise-free reception. If a current ratio, m, of 0.9 is used, the point of maximum fading would be moved to a distance of miles for a signal level of 0.2 mv./m.

Two physical embodiments of my improved antenna system are diagrammatically illustrated in Figures 4 and 5.

In Figure 4 the lower antenna section I is formed of a vertical metallic tube I2 in which is positioned a coaxial cable l3 containing a center conductor 14 and being grounded at its lower end. The upper end of tube [2 is connected to cable l3 at II, and the tube [2 is also connected to the cable H3 at the point l8 located one-quarter wavelength above the lower end of tube l2 which is insulated from the cable it by insulator 9. The upper antenna section is formed of two aligned tubular sections l0 and H positioned above the lower section I. Tubular sect on H] is supported on section II by an insulator 9a, and the lower end of this section is connected to the center conductor of the coaxial cable 13 at l5. The upper end of middle tubular section II is connected to the outer conductor of cable l3 as shown at 56, and the lower end of this section is supported on the cable l3 by the insulator 9b.

The two sections of the antenna are excited with energy from a transmission line 22 through a suitable phasing and power dividing network 2|, the lower section I being excited through connection 19 to the lower end of tube l2, and the upper section being excited by connection 20 to the lower end of the center conductor M of the coaxial cable 1 3,

In the arrangement shown in Figure 5 the lower section l of the antenna is formed of a vertical tubular portion 24 which is insulated from the ground and is excited by a connection 25 from the phasing network 2'! connected to transmission line 28. The upper section 2 of the antenna system is formed of a linear conductor 23, which may be in the form of a conducting tube or a tower, supported on insulator 9d and extending upwardly through the tube 24 and beyond the upper end of this tube to the distance B. The tube 24 is supported on the antenna part 23 by insulators 9 and 9c. Antenna section 2 is excited by a connection 26 to the lower end of part 23 from the phasing network 21.

It will be understood that by suitable adjustment of the phasing networks 2| and 21, the loop currents in the two antenna sections are adjusted to in-phase relation, and the amplitudes of the currents are also adjusted to the desired values as explained above.

It will be further understood that top loading of the top part of the elevated section 2 would still preserve the required electrical length yet reduce the physical length substantially.

I claim: I

1. An antenna system comprising a vertical antenna section extending above ground a distance of substantially 120 degrees of the operating wavelength, a second vertical antenna section arranged immediately above said first section and in alignment therewith, said second section having a radiating length of not more than one half wavelength, and the center of the radiating part of said second section being positioned above ground a distance of substantially 210 degrees of the operating wavelength, and means for exciting said sections by in-phase currents, the loop current in the upper section having an amplitude of substantially 0.7 of the amplitude of the loop current in the lower section.

2. An antenna system comprising a vertical antenna section extending above ground a distance of between 100 to 140 degrees of the operating wavelength, a second vertical antenna section arranged immediately above said first section and in alignment therewith, said second section having a length of not more than onehalf wavelength, and the center of said section being positioned above ground a distance of between 190 to 230 degrees of the operating wavelength, and means for exciting said sections by in-phase currents.

3. An antenna system according to claim 2 wherein the ratio of the loop current in the upper ill ial cable arranged within said tubular sections and having its lower end grounded, a connection from the center conductor of said cable to the lower end of the upper section, connections from the outer conductor of said cable to the upper ends of the middle and lower tubular sections, a connection between the outer conductor of said cable and the lower tubular section at a point one-quarter wavelength above ground, and exciting connections to the lower end of said lower section and to thelower end of the center conduct-or of said cable.

5. An antenna system comprising a vertical conductor supported in insulated relation with respect to ground and having a length of substantially 300 of the operating wavelength, a tubular conductor surrounding th lower section of said first conductor for a distance of substantially of the operating wavelength and being insulated therefrom, and exciting connections to the lower ends of said conductors for exciting in-phase currents in the tubular conductor and in the upper section of the first conductor.

6. A two-part vertical antenna comprising a lower radiating section insulated from ground and extending from ground to a point above ground a distance of substantially 129 degrees of the operating Wavelength, an upper radiating section arranged immediately above said lowor section and in alignment therewith, said upper section extending above said lower section a distance of not more than one-halal? wavelength, and the center of the radiating part of said upper section being positioned above ground a disv tance of substantially 210 degrees of the operating wavelength, and feed connections to the lower ends of said radiating sections for exciting in-phase currents therein, the loop current in the upper section having an amplitude of substantially 0.7 of the amplitude of the loop current in the lower section. I

CHARLES L. JEFFERS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Certificate of Correction Patent No. 2,502,155 March 28, 1950 CHARLES L. JEFFERS It is hereby certified that error appears in theprinted specification of the above numbered patent requiringcorreotion as follows:

Column 3, line 22, for (6): readf(6)=; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 8th day of August, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

